12/21/10: Rare Celestial Event

[Kit Sober]

As many as 1.5 billion people worldwide will be able to watch when the Earth's shadow creeps across the moon's surface early Tuesday morning, the first time in hundreds of years that a lunar eclipse will fall on the winter solstice.

With the full moon high in the winter sky, the lunar eclipse will be visible from four continents, with the best views from North America and Central America if weather permits, scientists say.

Heribert Proepper, APThe moon appears totally covered by shadow as the Earth passes between the moon and the sun during a lunar eclipse in January 2001."It's a really democratic event," Andrew Fraknoi, the chairman of astronomy at Foothill College in Los Altos Hills, told the San Francisco Chronicle, "because you don't need an expensive telescope or any other sophisticated equipment to enjoy the spectacle -- just your eyes or, if you like, a pair of binoculars."

Unlike a solar eclipse, eclipses of the moon can usually be observed anywhere in the hemisphere where the moon is above the horizon.

That means portions of this particular lunar eclipse could also be seen from northern and western Europe, and a small part of northeast Asia, including Korea and much of Japan. Totality will also be visible in its entirety from the North Island of New Zealand and Hawaii -- a potential viewing audience of about 1.5 billion people, according to Space.com.

Total lunar eclipses in northern winter are fairly common -– NASA says there have been three of them in the past ten years alone. However, a lunar eclipse falling precisely on the date of the solstice is quite unusual.

Geoff Chester of the U.S. Naval Observatory inspected a list of eclipses going back 2000 years for NASA.

"Since Year 1, I can only find one previous instance of an eclipse matching the same calendar date as the solstice, and that is 1638 DEC 21," Chester said, according to NASA. "Fortunately we won't have to wait 372 years for the next one ... that will be on 2094 DEC 21."

This year's event will take 3 hours and 38 minutes. The eclipse begins on Tuesday, Dec. 21, at 1:33 a.m. ET, according to NASA. At that time, Earth's shadow will appear as a dark-red mark at the edge of the moon. It will take about an hour for the mark to expand and cover the entire moon. Totality begins at 2:41 a.m. and lasts for 72 minutes.

During totality, the moon will be entirely immersed in the Earth's shadow, but the moon will not disappear from sight. Instead, it should appear to turn coppery red, as Earth's atmosphere refracts sunlight into the shadow.

Since the Earth's shadow is cone-shaped and extends out into space for about 844,000 miles, sunlight will be strained through a sort of "double sunset," all around the rim of the Earth, into its shadow and then onto the moon, according to Space.com.

If you're planning to dash out for only a quick minute, NASA recommends that you choose this one: 3:17 a.m. ET. That's when the moon will be in deepest shadow, displaying the most fantastic shade of copper red.

For more information on how often the December full moon coincides with the solstice, see this explanation of Greenwich mean time and the resulting time zones.

And for a graphic slide show of the lunar eclipse, see the website www.shadowandsubstance.com.